In a known example of a ball screw device for converting a rotational movement to a linear movement or converting a linear movement to a rotational movement, a nut member and a screw shaft are rotatably engaged with each other through an intermediation of a large number of balls. A rolling groove for the balls is spirally formed in an outer peripheral surface of the screw shaft at a predetermined lead, and also a through-hole into which the screw shaft is inserted is formed to the nut member. A load rolling groove facing the rolling groove of the screw shaft is formed in an inner peripheral surface of the through-hole. With the load rolling groove of the nut member and the rolling groove of the screw shaft facing each other, a spiral load ball path is formed between the nut member and the screw shaft, and the balls are rolled while bearing a load acting between the nut member and the screw shaft in the load ball path. With this structure, a relative spiral movement can be performed between the nut member and the screw shaft. Further, the nut member is provided with a no-load ball path for communicatively coupling the both ends of the load ball path, and the balls rolled in the load ball path are returned to the load ball path through the no-load ball path so as to circulate. That is, the nut member is provided with an infinite circulation path for the balls, and the balls circulate in the load ball path and the no-load ball path, whereby the nut member can be continuously moved with respect to the screw shaft.
As methods of providing the nut member with an infinite circulation path for the balls, there are known three dominant methods as follows. The first method is a so-called return tube method involving mounting a return tube formed in a substantially U-shape to the nut member. In this method, the return tube is mounted to the nut member so as to cross over several grooves of the spiral load ball path, and the balls are caused to circulate from one end to the other end of the load ball path through the no-load ball path provided to the return tube (JP 2005-003106 A and the like).
Further, the second method is a so-called deflector method involving embedding a die referred to as a deflector in a nut member. The deflector is provided with a ball returning groove so as to face the screw shaft and cross over ungrooved portions of the screw shaft, and the ball returning groove couples the end portions to each other of a single groove of the load ball path around the screw shaft. With this structure, when the balls having been rolled in the load ball path reach the mounting position of the deflector, the balls are guided into the ball returning groove so as to depart from the rolling groove of the screw shaft and climb over the ungrooved portion of the screw shaft, and then returned to the inlet of the load ball path (JP 2006-038099 A and the like).
Further, the third method is a so-called end cap method. In this end cap method, the nut member is passed through so as to form the ball returning path in the axial direction and is provided with end caps including direction change paths for the balls at both axial ends of the nut member, and the balls having been rolled in the load ball groove are guided into the ball returning path of the nut member via the direction change paths. That is, the end portions of the load ball path are coupled to the end portions of the no-load ball path by means of the direction change paths provided with the end caps, whereby the infinite circulation path for the balls is realized (JP 2005-042796 A and the like).    Patent Document 1: JP 2005-003106 A    Patent Document 2: JP 2006-038099 A    Patent Document 3: JP 2005-042796 A